A zinciferous plated steel sheet having a zinciferous plating layer comprising zinc or a zinc alloy formed on at least one surface of the steel sheet is widely employed in various sectors of industry because of the excellent corrosion resistance. When using such a zinciferous plated steel sheet as a material for a business machine such as a copying machine, an audio equipment, a home electrical appliance or an automobile body, a press forming is applied to the zinciferous plated steel sheet. When using the zinciferous plated steel sheet as a material for a motor cover or a cartridge-type tank, a drawing forming is applied to the zinciferous plated steel sheet.
As compared with a cold-rolled steel sheet, however, the zinciferous plated steel sheet has a problem of a lower press-formability. More particularly, frictional resistance of the zinciferous plated steel sheet against a forming die during the press forming is larger than frictional resistance of the cold-rolled steel sheet against the forming die. The zinciferous plated steel sheet has therefore a poor lubricity. As a result, a severe press forming, if applied to the zinciferous plated steel sheet, causes a peeloff or a galling in the zinciferous plating layer.
Demand is recently increasing for a zinciferous plated steel sheet, which has a chromate film formed on the zinciferous plating layer by applying a chromating treatment to the zinciferous plated steel sheet and has a more excellent corrosion resistance. The zinciferous plated steel sheet having the chromate film formed on the zinciferous plating layer is excellent in corrosion resistance in the form of a flat sheet, i.e., corrosion resistance before a press forming. However, when applying a press forming to the zinciferous plated steel sheet having a chromate film, the poor press-formability as described above causes a peeloff of the chromate film or a blackish discoloration of the surface of the chromate film. Corrosion resistance and external appearance of the press-formed zinciferous plated steel sheet are thus deteriorated.
As a means to solve the above-mentioned problems, the conventional practice has been to improve lubricity of the zinciferous plated steel sheet by applying a lubricant oil onto the surface thereof prior to press-forming the zinciferous plated steel sheet.
However, application of the above-mentioned lubricant oil onto the surface of the zinciferous plated steel sheet has the following problems:
(a) The applying operation of the lubricant oil causes a complicated manufacturing process; PA1 (b) The lubricant oil stains an operation site, thus polluting a working environment; and PA1 (c) If press forming conditions are severe, a galling may be produced during the press forming even after applying the lubricant oil, thus causing deterioration of corrosion resistance of the press-formed zinciferous plated steel sheet. PA1 (1) A zinciferous plated steel sheet excellent in lubricity and corrosion resistance, disclosed in Japanese Patent Provisional Publication No. 61-60,886 published on Mar. 28, 1986, which comprises: a steel sheet; a zinciferous plating layer formed on at least one surface of said steel sheet; a chromate film formed on said zinciferous plating layer, said chromate film having a weight within a range of from 5 to 200 mg/m.sup.2 per surface of said steel sheet; and a composite silicate film formed on said chromate film, which contains a water-dispersing internal lubricant and comprises a water-soluble or water-dispersing organic resin and a water-dispersing silica, said composite silicate film having a thickness within a range of from 0.3 to 3.0 .mu.m (hereinafter referred to as the "prior art 1"). PA1 (2) A zinciferous plated steel sheet excellent in workability and lubricity, disclosed in Japanese Patent Provisional Publication No. 62-289,275 published on Dec. 16, 1987, which comprises: a steel sheet; a zinciferous plating layer formed on at least one surface of said steel sheet; and a thermosetting resin film formed on said zinciferous plating layer, which has fluorine resin powder exposed on the surface of said thermosetting resin film (hereinafter referred to as the "prior art 2"). PA1 (3) A zinciferous plated steel sheet excellent in corrosion resistance and lubricity, disclosed in Japanese Patent Provisional Publication No. 1-110,140 published on Apr. 26, 1989, which comprises: a steel sheet; a zinciferous plating layer formed on at least one surface of said steel sheet; a chromate film formed on said zinciferous plating layer, said chromate film having a weight within a range of from 3 to 200 mg/m.sup.2 per surface of said steel sheet; and an organic resin film formed on said chromate film, which comprises colloidal silica in an amount of from 5 to 40 wt. %, a solid internal lubricant in an amount of from 3 to 20 wt. % surface-treated with a titanate coupling agent, an epoxy resin in an amount of from 2 to 30 wt. %, and the balance being acrylic resin (hereinafter referred to as the "prior art 3"). PA1 (4) A zinciferous plated steel sheet excellent in blacking resistance, disclosed in Japanese Patent Provisional Publication No. 2-140,294 published on May 29, 1990, which comprises: a steel sheet; a zinciferous plating layer formed on at least one surface of said steel sheet; a chromate film formed on said zinciferous plating layer; and a montan wax oxide film formed on said chromate film (hereinafter referred to as the "prior art 4"). PA1 a steel sheet; PA1 a zinciferous plating layer formed on at least one surface of said steel sheet; PA1 a chromate film formed on said zinciferous plating layer, said chromate film having a weight within a range of from 5 to 200 mg/m.sup.2 as converted into metallic chromium per surface of said steel sheet; and PA1 a resin film formed on said chromate film by applying a resin onto said chromate film and then curing same, said resin film having a thickness within a range of from 0.3 to 3.0 .mu.m; PA1 said resin film comprises a solvent-base thermosetting resin, a solid internal lubricant and a rust-preventive pigment; PA1 (A) urethane prepolymer containing a hydroxyl group and having the following chemical composition: PA1 (B) at least one of a block polyisocyanate compound and an amino resin as a curing agent; PA1 said solid internal lubricant comprises a polyethylene resin, and the content ratio of said solid internal lubricant is within a range of from 1 to 30 weight parts relative to 100 weight parts of a solid component in said solvent-base thermosetting resin; and PA1 said rust-preventive pigment comprises at least one of a chromate compound or silica, and the content ratio of said rust-preventive pigment is within a range of from 3 to 30 weight parts relative to 100 weight parts of said solid component in said solvent-base thermosetting resin.
As a zinciferous plated steel sheet which solves the above-mentioned problems and is excellent in press-formability and corrosion resistance even without applying a lubricant oil onto the surface thereof, the following zinciferous plated steel sheets are proposed:
The prior art 1 has the following problems:
Under severe press-forming conditions, frictional heat produced during the press forming causes deterioration of the composite silicate film formed on the chromate film, thus resulting in sticking of the composite silicate film to the forming die or blackening of the composite silicate film. As compared with the conventional zinciferous plated steel sheet applied with a lubricant oil on the surface thereof, therefore, the zinciferous plated steel sheet of the prior art 1 may become poorer in corrosion resistance and external appearance after the press forming.
The prior art 2 has the following problems:
The fluorine resin powder exposed on the surface of the thermosetting resin film may peel off during the press forming, and accordingly pinholes tend to occur in the zinciferous plating layer. In addition, the thickness of the thermosetting resin film may become non-uniform because of the fluorine resin powder exposed on the surface of the thermosetting resin film.
The prior art 3 has the following problems:
Under severe press-forming conditions, as in the case of prior art 1, frictional heat produced during the press forming causes deterioration of the organic resin film formed on the chromate film, thus in turn causing sticking of the organic resin film to the forming die or blackening of the organic resin film. As compared with the conventional zinciferous plated steel sheet applied with a lubricant oil on the surface thereof, therefore, the zinciferous plated steel sheet of the prior art 3 may become poorer in corrosion resistance and external appearance after the press forming.
The prior art 4 has the following problems:
The montan wax oxide film formed on the chromate film has a low softening point. Under severe press-forming conditions, therefore, frictional heat produced during the press forming causes peeloff of, or damage to, the montan wax oxide film. As compared with the conventional zinciferous plated steel sheet applied with a lubricant oil on the surface thereof, therefore, the zinciferous plated steel sheet of the prior art 4 may become poorer in corrosion resistance and external appearance after the press forming.
In addition to the above-mentioned prior arts 1 to 4, there are proposed zinciferous plated steel sheets each having a resin film comprising one of various resins and one of various solid internal lubricants formed on a chromate film. In these conventional zinciferous plated steel sheet, however, deterioration of the resin film is unavoidable under severe press-forming conditions.
Under such circumstances, there is a strong demand for development of a zinciferous plated steel sheet excellent in press-formability and corrosion resistance, which permits the prevention of peeloff of, or damage to, the resin film formed on the chromate film on the zinciferous plating layer, even under severe press-forming conditions, but such a zinciferous plated steel sheet has not as yet been proposed.
An object of the present invention is therefore to provide a zinciferous plated steel sheet excellent in press-formability and corrosion resistance, which permits the prevention of peeloff of, or damage to, the resin film formed on the chromate film on the zinciferous plating layer, even under severe press-forming conditions.